Method for Detecting the Sealed State of a Fuel Cell

ABSTRACT

Procedure for detecting the sealing state of a fuel cell stack in which, as soon as the fuel cell stack is considered to be extinguished, the sum of the pressures in the anode circuit and in the cathode circuit equal to P 1  is recorded. After an additional period of time of 180 seconds, the sum of the pressures in the anode circuit and in the cathode circuit equal to P 2  is recorded. If P 2  is less than P 1 , an alarm is triggered.

FIELD OF THE INVENTION

The present invention relates to fuel cell stacks, in particular, butnot exclusively, to fuel cell stacks of the type having an electrolytein the form of a polymeric membrane (i.e. of the PEFC (PolymerElectrolyte Fuel Cell) type).

PRIOR ART

It is known that fuel cell stacks produce electrical energy directly viaan electrochemical redox reaction using hydrogen (the fuel) and oxygen(the oxidant) without passing via a mechanical energy conversion step.This technology seems promising, especially for motor vehicleapplications. A fuel cell stack comprises in general the seriescombination of unitary elements each consisting essentially of an anodeand a cathode separated by a polymeric membrane allowing ions to passfrom the anode to the cathode.

It is very important to have a precise evaluation of the sealing stateof the fuel cell, that is to say the sealing of the gas circuit at theanode (fuel gas circuit) and the sealing of the oxidant gas circuit (gascircuit at the cathode). This is because a gas leak will inevitablydisturb the operation of the fuel cell stack and pollute the environmentthereof, in particular if this is a fuel gas leak. As a consequence, thefuel cell stack may suffer a loss of power, a reduction in efficiency orpremature ageing, and the operating conditions, for safe operation, mayeven be compromised.

Patent application WO 2003/061046 discloses an extinction procedure fora polymer electrolyte membrane fuel cell operating with air as oxidantgas. The procedure disclosed consists in maintaining the pressuredifference between the anode and the cathode below an acceptable level.To do so, the air feed is maintained during extinction and the airpressure is controlled so as to follow the drop in pressure on thehydrogen side. However, maintaining the air feed risks causing ahydrogen starvation, which for stack survival is quite worrying.Moreover, the above document teaches no means for observing the sealingstate of a fuel cell stack.

The objective of the present invention is to be able to observe thesealing state of the fuel cell stack with respect to atmosphere, inparticular after each extinction, in order to monitor and diagnose afuel cell stack, without it being necessary to add equipment just forproviding a monitoring function, that is to say equipment that is in noway necessary for the normal operation of the fuel cell stack.

BRIEF DESCRIPTION OF THE INVENTION

The invention proposes a procedure for detecting the state of a fuelcell stack, the fuel cell stack being formed by stacking electrochemicalcells each having an anode and a cathode on either side of a polymericion-exchange membrane, the fuel cell stack having a fuel gas supplysystem on the anode side of the electrochemical cells and an oxidant gassupply system on the cathode side of the electrochemical cells, theprocedure consisting, upon each shut-down of the fuel cell stack, inmeasuring the dynamic behaviour whereby the pressure in the anodecircuit and the pressure in the cathode circuit evolve and, when saiddynamic behaviour shows pre-identified characteristic signs, a warningsignal indicating that the fuel cell stack requires inspection isactivated.

The applicant has in fact observed that, when said dynamic behaviourshows pre-identified characteristic signs, precise examples of whichwill be given below, the fuel cell stack shows a loss of sealing, whichmay impair safety and reduce the efficiency and durability. Aninspection is then necessary in order to examine more closely thedegradation of the fuel cell stack so as thereafter to be able to takeappropriate measures (repair or scrapping).

According to one aspect of the invention, to obtain an evaluation of thestate of a fuel cell stack, a mathematical function combining thepressure in the cathode circuit with the pressure in the anode circuitis constructed. The variation over the course of time of this function,as a measurement of said dynamic behaviour whereby the pressure in theanode circuit and the pressure in the cathode circuit evolve, isobserved.

Preferably, to implement the invention, the fuel cell stack comprises apressurized oxygen supply, coming from an oxygen storage tank, and adevice for filling with pressurized atmospheric air, together with arecycling circuit connected to the outlet of the cathode circuit of thefuel cell stack.

In the rest of the description, the invention is illustrated byconsidering a fuel cell stack supplied with pure oxygen as oxidant gas.This aspect however is not limiting, it being possible for the inventionalso to apply to fuel cell stacks supplied with ambient air. Theembodiment described (supply with pure oxygen) is conducive to thecompactness of the given fuel cell stack, this constituting a favourableembodiment for applications in transport vehicles, in particular inmotor vehicles.

This application case means in practice designing a fuel cell stack thathas substantially the same internal volume for the fuel gas supplycircuit on the anode side of the electrochemical cells and for theoxidant gas supply circuit on the cathode side of the electrochemicalcells. In this case, one appropriate simple mathematical function is thesum of the pressure in the cathode circuit and the pressure in the anodecircuit, another function being the average of the pressure in thecathode circuit and the pressure in the anode circuit. Of course, itshould be realized that, to implement one or other of the aforementionedmathematical functions (sum or average), the gas circuits on the anodeside and on the cathode side should contain the same number of moles. Ifthis is not the case, a person skilled in the art will know how to applyrelevant adaptation coefficients or, more generally, will know how tochoose a relevant mathematical function for collectively monitoring thesealing of both gas circuits of a fuel cell stack with respect to theatmosphere.

According to another aspect of the invention, the measurement orobservation or evaluation of said dynamic behaviour starts as soon asthe stack is completely shut down with residual pressures in the anodeand cathode circuits that differ from atmospheric pressure. In thiscase, the procedure for detecting the state of a fuel cell stack is suchthat the pressure variation in said circuits over a predetermined timeperiod t_(c) is measured.

According to one aspect of the invention, to evaluate said dynamicbehaviour, instead of measuring the pressure difference after a giventime, the time to reach a given pressure difference is measured. Theinvention of course covers other ways of evaluating said dynamicbehaviour.

Preferably, the procedure for detecting the sealing state of a fuel cellstack as explained above is preceded by a procedure for shutting downsaid fuel cell stack, the latter delivering an electrical voltage to anelectrical power line (10), the shut-down procedure comprising thefollowing actions:

-   -   (i) the supply of fuel gas and oxidant gas is cut off;    -   (ii) current continues to be drawn as long as an appropriate        indicator indicates that the oxidant gas in the oxidant gas        supply system has not been sufficiently consumed; and    -   (iii) nitrogen-enriched gas is injected into the oxidant gas        supply system.

The actions (i), (ii) and (iii) could all be concomitant. To make thefollowing description better understood, the actions (ii) and (iii) aresuccessive steps, the two actions (i) and (ii) being concomitant. It isalso useful to provide, after the action (iii), a fuel gas suction step,as is also shown in the description of the shut-down procedureillustrating the invention.

By virtue of the shut-down procedure proposed above, the hydrogendiffuses into the cathode only very slowly through the polymericion-exchange membrane and after extinction, that is to say after all theoxygen has been consumed and the cathode circuit has been filled withnitrogen. Oxygen and hydrogen therefore never cohabit in significantamount. The hydrogen supply is interrupted right from the start of theprocedure simultaneously or almost simultaneously with the cutting-offof the oxidant gas supply. Although the action of interrupting the fuelgas supply could be somewhat delayed relative to the action ofinterrupting the oxidant gas supply, it may not be significantlydelayed. The following description is limited only to the case in whichthe supply of oxidant gas and the supply of fuel gas are interruptedsimultaneously, which is the simplest procedure to control and givesentirely satisfactory results. All the residual hydrogen at the anode isparsimoniously used to guarantee the desired H₂/N₂ mixture.

It should be noted that the shut-down procedure proposed above extendsto a fuel cell stack in which the additional fuel gas accumulationchamber could be placed at any point in the fuel gas supply circuit,that is to say at any point between the cut-off valve and the fuel cellstack, even in the recycling circuit, or in the circuit between thewater separator and the ejector. However, it is advantageous to place itat a point in the circuit where the pressure is highest so as to reducethe volume thereof, as specified in the description of the above fuelcell stack.

In any case, as regards the electrolyte, the invention applies to fuelcell stacks of the type having an electrolyte in the form of a polymericmembrane (that is to say one of the PEFC type). The electricitygenerator and the shut-down procedure described below prove to beparticularly suitable for being installed and implemented in a motorvehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The rest of the description serves to make all the aspects of theinvention clearly understood by means of the appended drawings in which:

FIG. 1 is a diagram of an electricity generator using a fuel cell stacksupplied with pure oxygen;

FIG. 2 shows the behaviour of various parameters during the extinctionof a fuel cell stack;

FIG. 3 shows the behaviour of the pressures after an extinction andillustrates the principle of measuring the sealed state of the stack.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

For safety reasons, fuel cell stacks are generally equipped with an H₂cut-off valve which remains closed during shut-downs. In this case, itis not possible to draw H₂ into the tank during the extinctionprocedure. The fuel cell stack must therefore function with only theresidual hydrogen in its channels, ducts, internal dehumidifyingreservoirs and other components of the supply line going from the safetyvalve to the actual fuel cell stack, these components being denotedhereafter in general as the supply circuit for the fuel cell stack.

FIG. 1 shows a fuel cell stack 1 of the type having an electrolyte inthe form of a polymeric membrane (i.e. of the PEFC or PEM (protonexchange membrane) type). The fuel cell stack 1 is supplied with twogases, namely the fuel (hydrogen stored or generated on board thevehicle) and the oxidant (pure oxygen), which gases supply theelectrodes of the electrochemical cells. An electrical load 14 isconnected to the fuel cell stack 1 via an electrical line 10. Tosimplify matters, FIG. 1 shows only the gas circuit components usefulfor understanding the invention.

Description of the Anode Circuit

The installation comprises a fuel gas supply circuit 11 on the anodeside. A pure hydrogen (H₂) tank 11T is visible, this being connected tothe inlet of the anode circuit of the fuel cell stack 1 by means of asupply line that passes via a cut-off valve 110, then via an ejector 113and then via a fuel gas supply channel 11A terminating at the cathodes.A pressure probe 111 is installed in the supply channel 11A just beforethe inlet into the fuel cell stack 1. Forming part of the hydrogen(fuel) supply circuit 11 is a circuit 11 R for recycling the hydrogennot consumed by the fuel cell stack, said circuit being connected to theoutlet of the anode circuit of the fuel cell stack 1. A water separator114 is installed in the recycling circuit 11R. The ejector 113 and arecirculating pump 115 recycle the unconsumed hydrogen and mix it withfresh hydrogen coming from the tank.

An additional fuel gas accumulation chamber 116 is also visible, thisbeing placed on the piping of the fuel gas supply circuit 11, betweenthe cut-off valve 110 and the pressure regulating valve 117. Theadditional accumulation chamber is, in this preferred embodiment, placedat the point where the pressure is highest in the supply circuit, so asto lessen the volume thereof or, for the same volume, to stock a largeramount of hydrogen. It should be noted that the additional fuel gasaccumulation chamber 116 could be placed at any point in the fuel gassupply circuit, that is to say at any point between the cut-off valve110 and the fuel cell stack 1, even in the recycling circuit 11R or inthe circuit between the water separator 114 and the ejector 113.However, it is advantageous to place it at a point in the circuit wherethe pressure is highest, so as to reduce the volume thereof.

A suction pump 119 and a cut-off valve 118 that are installed on a lineventing to atmosphere and connected below the water separator 114, canalso be seen. The connection at this point, shown in FIG. 1, makes itpossible, by controlling the cut-off valve 118, to provide threefunctions, namely water discharging, purging and hydrogen suction.However, this embodiment detail is not limiting. To provide the morespecific hydrogen suction function of the present invention, the linehaving the cut-off valve 118 could be branched off the line connectingthe separator 114 to the recirculating pump 115.

A hydrogen concentration sensor C11 may advantageously be inserted intothe anode circuit so as to check there is no hydrogen starvation duringthe extinction procedure and, where appropriate, to limit the injectionof air by the booster pump (see description of the cathode circuit),which may occur for example if the hydrogen pressure is abnormally lowand does not ensure the sufficient amount of hydrogen for completing theextinction procedure. Such a hydrogen sensor C11 is installed as shownin FIG. 1.

Description of the Cathode Circuit

The installation also includes an oxidant gas supply circuit 12 on thecathode side. A pure oxygen (O₂) tank 12T is visible, this beingconnected to the inlet of the cathode circuit of the fuel cell stack 1by means of a supply line that passes via a cut-off valve 120, then viaa pressure regulating valve 127, then via an ejector 123 and then via anoxidant gas supply channel 12A terminating in the cathodes. A pressureprobe 121 is installed in the supply channel 12A just before the inletinto the fuel cell stack 1. Forming part of the oxygen supply circuit 11is a circuit 12R for recycling the oxygen not consumed by the fuel cellstack, connected to the outlet of the cathode circuit of the fuel cellstack 1. A water separator 124 is installed in the recycling circuit12R. The ejector 123 and a recirculating pump 125 recycle the unconsumedoxygen and mix it with fresh oxygen coming from the tank.

A purge valve 122 is connected to the bottom of the water separator 124.This valve thus provides two functions, removal of the water and ventingof the oxygen circuit to atmosphere. As a variant, this purge valve 122could be connected just at the gas outlet of the fuel cell stack 1,branched off the line between the fuel cell stack 1 and the waterseparator 124, if it is desired to vent the oxygen circuit to atmosphereindependently of draining the water in the water separator 124. It goeswithout saying that, in all cases, the function of draining water fromthe water separator 124 and from the water separator 114 must beensured.

The fuel cell stack according to the invention includes a filling device12N, for filling the cathode circuit with pressurized atmospheric air.The filling device 12N comprises the following components: a linestarting with an air intake orifice 126 and, installed on said line, acut-off valve 128 and a booster pump 129, the line terminating in theoxygen supply circuit, just upstream of the fuel cell stack 1. We shouldpoint out that the atmospheric air filling device 12N could terminate atany point in the loop of the oxidant gas supply circuit 12, said loopbeing formed by the recycling circuit 12R and by the line connecting theejector 123 to the fuel cell stack 1.

Description of a Preferred Extinction Procedure

The procedure described below makes it possible to extinguish the fuelcell stack so as to guarantee storage with a hydrogen/nitrogen mixturetherein, without requiring a nitrogen bottle. This procedure isrecommended because it terminates by naturally leaving the fuel cellstack with a pressure differential with respect to atmospheric pressuresufficient to be able to carry out the stack sealing state measurement.In addition, the procedure is conducive to stable conditions in terms ofthe nature of the gases and the temperature, thereby guaranteeing betterrepeatability of the stack sealing state measurement.

The shut-down procedure is essentially made up of 3 phases, resultingfrom various commands that are explained:

-   -   1^(st) phase: residual oxygen consumption phase, which occurs        upon cutting off the fuel gas supply and oxidant gas supply, and        by drawing a current I_(s) at the terminals of the fuel cell        stack. This current draw I_(S) is maintained as long as an        appropriate indicator indicates that the oxidant gas in the        oxidant gas supply system has not been sufficiently consumed. An        appropriate indicator is for example the pressure in the cathode        circuit;    -   2^(nd) phase: neutralization phase that occurs when filling the        cathode circuit with nitrogen. In the embodiment described here,        the nitrogen is that of the atmospheric air. Forced injection of        atmospheric air then takes place, thereby again introducing a        little oxygen, the consumption of which must be controlled; and    -   3^(rd) phase, which is optional, during which, after the        electrochemical processes have been completely shut down, any        excess fuel gas is forcibly removed (here, forced suction of the        excess hydrogen). It should be emphasized that, by virtue of the        invention, this suction takes place only after the fuel cell        stack has been brought into a state in which the precautions for        avoiding insufficient supply of hydrogen, the serious        consequences of which are known, have been taken.

FIG. 2 illustrates the sequence of the three phases during a shut-downactually measured on a fuel cell stack comprising twenty cells having anactive area of 300 cm², operating with pure oxygen. The x-axis indicatesthe time in seconds, with as reference (0) the instant when theshut-down procedure starts. This figure shows the variation of thefollowing quantities as a function of time during a shut-down withnitrogen generation:

-   -   Curve 1, the y-axis of which is labelled “stack current [A]”,        showing the current drawn from the fuel cell stack, expressed in        amps;    -   Curve 2, the y-axis of which is labelled “stack voltage [V]”,        showing the total electrical voltage across the terminals of the        fuel cell stack, expressed in volts;    -   Curve 3, the y-axis of which is labelled “pressure out [bar]”,        showing the pressure within the anode compartment (hydrogen:        solid line) and in the cathode compartment (oxygen: dotted        line), expressed in bara; and    -   Curve 4, the y-axis of which is labelled “H2 concentration [%]”,        showing the hydrogen concentration in the anode compartment        (hydrogen: solid line) and in the cathode compartment (oxygen:        dotted line), expressed in %.

During the first phase of the extinction (0 to 35 s, marked “oxygendepletion” in FIG. 2), starting from the moment when the oxygen supplyis cut off (by closing the cut-off valve 120 at the same instant thatthe cut-off valve 110 is closed, cutting off the hydrogen supply), theresidual pure oxygen in the fuel cell stack is consumed by drawing acurrent. As the first curve indicates, this current is firstlyestablished at 50 A and it is then reduced at the same time that some ofthe cells of the fuel cell stack start to drop in voltage and is finallystopped at 35 s when the voltage of the fuel cell stack approaches 0 V.The third curve indicates that the pressure in the oxygen compartmentdrops to less than 500 mbara (as is usual in the field of fuel cellstacks, “mbara” means “millibar absolute”, the final letter “a” denoting“absolute”). However, despite the consumption associated with currentproduction, the hydrogen pressure remains at 1.75 bara because of thepresence of the additional fuel gas accumulation chamber 116.

As already emphasized in the introductory part of this patentapplication, the extinction procedure according to the invention mayalso apply to fuel cell stacks supplied with ambient air. To implementthe shut-down procedure proposed by the invention for a fuel cell stacksupplied with air, unlike the usual scheme for supplying such a fuelcell stack, the oxidant gas circuit must include a loop for circulatingthe air not consumed by the fuel cell stack, at least during theshut-down procedure. Therefore forming part of the air supply circuit 11is a recycling circuit 12R for recycling the air not consumed by thefuel cell stack, connected to the outlet of the cathode circuit of thefuel cell stack 1, before a return and a direct connection (with noejector nor a water separator, which are unnecessary in thisconfiguration) to the supply line.

Let us return to the description of the shut-down procedure for a fuelcell stack supplied with pure oxygen. At the time 35 s (“35” on the timeaxis in FIG. 2), the air booster pump 129 is activated in order topressurize the cathode circuit to a constant pressure of 2.2 bara(parameter 1), which is reached at 50 s. The oxygen thus supplied causesthe fuel cell stack voltage to rise again. A current is drawn once moreuntil the voltage of the fuel cell stack again becomes zero. In themeantime, the booster pump 129 is monitored so as to keep a constantpressure.

Incidentally, it should be remembered that all the curves detailed belowrelate to a fuel cell stack supplied with pure oxygen as oxidant, thenitrogen-enriched gas being the atmospheric air. However, it should bepointed out that, on the one hand, the nitrogen-enriched gas could bepure nitrogen and that, of course, in this case, the curves would have adifferent appearance after the instant “35 seconds” since the nitrogeninjection would not be accompanied by a new supply of oxygen.

Let us return to the case described, namely the case of a fuel cellstack supplied with pure oxygen as oxidant. As the current is beingconsumed, the air present at the cathode becomes increasingly depletedin oxygen before finally containing only predominantly nitrogen, asrevealed by the voltage across the terminals of the fuel cell stackbecoming zero at the 65 s instant.

At this moment (65 seconds after the oxygen (120) and hydrogen (110)supplies have been cut off), the air booster pump 129 is stopped and thehydrogen suction pump 119 is activated, so as to remove the excesshydrogen. The suction pump 119 remains activated until the hydrogenpressure reaches 0.5 bars (parameter 2). This pressure is reached at the75 s instant. The procedure is then terminated, the booster pump 129 andthe suction pump 119 are stopped and the cut-off valves 118 and 128 areclosed.

Throughout the entire extinction procedure, the recirculating pump 125on the cathode side is kept in operation so as to ensure goodhomogeneity of the gas and to ensure complete consumption of the oxygen,preventing the appearance of zones with a higher oxygen concentrationlocally. The recirculating pump 115 on the anode side is also kept inoperation so as to avoid any local hydrogen starvation. Throughout theextinction period, hydrogen starvation is avoided as the hydrogenconsumption shown by the fourth curve indicates. The concentrationremains above 85% in the anode circuit until the 65 second instant, whenthe hydrogen suction starts.

In the procedure described above, the first two phases (residual oxygenconsumption and neutralization by means of nitrogen injection) takeplace in succession. However, they could just as well be concomitant.For greater rapidity of extinction, it is desirable to make them occursimultaneously. The final phase (excess hydrogen suction) is not alwaysessential. The hydrogen buffer tank may in fact be designed so that theprocedure terminates with the desired amount of hydrogen as explainedbelow.

The internal volume of the fuel gas supply circuit 11 is designed to begreater than the internal volume of the oxidant gas supply circuit 12and, in normal operation, the pressure within the oxidant gas supplycircuit 12 and the pressure within the fuel gas supply circuit 11 aresuch that, given the internal volume of the oxidant gas supply circuit12 and the internal volume of the fuel gas supply circuit 11, the numberof moles of fuel gas always available at the start of the extinctionprocess in the fuel gas supply circuit is greater than or equal to twicethe number of moles of oxygen consumed in the oxidant gas supply circuitduring the entire extinction procedure, that is to say until the cathodecircuit is essentially filled with nitrogen at the desired pressure.

Thus, in a simple adaptation to be calculated and implemented, it ispossible to ensure that the fuel gas supply circuit always containssufficient gas, for the extinction of the fuel cell stack to result fromthe oxygen in the oxidant gas supply circuit being exhausted.

Let us see how to calculate the volumes of the anode circuit 12 and thecathode circuit 11. Let m_(o2) be the amount of oxygen, expressed forexample in moles, that has to be completely consumed over the entireextinction. This is the residual oxygen in the cathode circuit at thestart of extinction less the amount that it is possible to purge, plusthe amount which is introduced with the air introduced by the boosterpump 129 to generate the nitrogen.

Since the gas consumption is twice as high on the hydrogen side, thevolumes of the anode and cathode circuits must be sized so as toguarantee that:

m _(h2)2×m _(o2)+res_(h2)

where m_(h2) is the amount of hydrogen, expressed in moles, available atthe start of extinction in the internal volume of the fuel gas supplycircuit (pipes, channels, bipolar plates, supply line downstream of thecut-off valve 110) and res_(h2) is the desired amount of residualhydrogen, also expressed in moles. The amount of hydrogen m_(h2) finallynecessary will be obtained by adjusting the volume of the additionalfuel gas accumulation chamber 116.

The amounts m_(o2) and m_(h2) are admittedly linked to the volume of thecorresponding circuits that it is necessary to dimension, but they alsodepend on the pressure prevailing therein. This is a simplified approachsince it would normally be necessary also to take account of thetemperature of the gas and the nonlinearity of the hydrogen density as afunction of the pressure. However, taking the pressure into accountproves to be sufficient for the desired precision. The volumes have tobe calculated for the most unfavourable pressure and temperatureconditions that may be encountered, that is to say the minimum possiblepressure in the hydrogen circuit at the start of extinction and themaximum possible residual pressure in the oxygen circuit.

However, in the case of a supply pressure variation, the execution ofthe procedure with an excess of hydrogen and a final suction guaranteesthat there is no hydrogen starvation and also better reproducibility ofthe final conditions.

Referring to FIG. 3, giving the variation in the pressures on the anodeside and the cathode side over a longer period than in FIG. 2,comprising the extinction procedure described above and extending forsome 10 minutes or so thereafter. At the end of the extinction procedure(about 100 seconds after the oxygen and hydrogen supplies have been cutoff), the pressures in the anode compartment (continuous line) and thecathode compartment (long dotted line) are different from atmosphericpressure, in the example above 2.2 bara at the cathode and 0.5 bara atthe anode. It may be seen that these two pressures converge on a commonequilibrium pressure which is established at about 1.3 bara after 10minutes, because of the permeability of the membranes separating the twocompartments. However, it may be seen that the algebraic sum of the twopressures represented by the short dotted curve remains substantiallyconstant, provided that the stack is properly sealed with respect to theatmosphere. Thus, if a check is made as soon as the fuel cell stack isconsidered to have been extinguished, the sum of the pressures in theanode circuit and the cathode circuit equal to P₁ is recorded.Thereafter, the sum of the pressures, in the anode circuit on the onehand and the cathode circuit on the other, will remain substantially thesame, a sign that said circuits are properly sealed. After an additionalperiod of time t_(c) of 180 seconds, the sum of the pressures in theanode circuit and the cathode circuit equal to P₂ is recorded, where P₂is practically the same as P₁.

Thus, it may be seen that a procedure can be carried out for detectingthe state of a fuel cell stack in which the variation over the course oftime of said mathematical function, here the algebraic sum, is recorded,and an alarm is triggered when the pair of values consisting of the timeand the value of the function associated with said time exceeds apredetermined threshold. The pressure variation in said circuits over apredetermined time period t_(c) is measured and a mathematical functioncombining the values of the pressure in said circuits is calculated. Awarning is then given when the function is below a warning threshold atthe end of a predetermined time period.

In practice, for a fuel cell stack of the PEFC type having a totalcumulative active area of around 5000 cm², a volume of the anode andcathode circuits of the order of 1 litre and an average pressuredifferential relative to atmosphere at the end of extinction of about500 mbar, experimental observations provide the following good practicerules: a warning threshold is to be activated when the sum of the anodeand cathode pressures drops by 240 mbar in 3 minutes at a temperature of70° C. Beyond this, a shut-down threshold may be set when the sum of theanode and cathode pressures drops by 300 mbar in 3 minutes at atemperature of 70° C. When the shut-down threshold level is reached,functioning of the stack is prohibited (for example by a program in thecentral control unit) and a maintenance action is made necessary. Theseals at various places in a fuel cell stack are often the cause ofsealing deficiencies. To identify the origin of the leak during themaintenance operation, one method consists in pressurizing the stackwith an inert gas, preferably nitrogen, at a pressure close to thenominal operating pressure, and then in spraying a foaming productcommonly used for detecting leaks onto the outside of the stack in orderto locate the origin of the leak.

As regards the nature of the gases and the pressure, the shut-downprocedure described above provides correct conditions and allows verygood repeatability to be achieved, namely pure hydrogen at the anode ata pressure of 500 mbara and pure nitrogen at the cathode at a pressureof 2.2 bars. It is necessary to ensure that the gases present do notallow any electrochemical activity, which would make thepressure-variation measurement of the state of the stack inoperable. Itis also clearly understood that the anode and cathode circuits must beclosed during the measurement so that there is no gas exchange with theambient environment or with the tanks, something which would completelyfalsify the measurement of the stack state. This means that the valves128, 122, 120, 127, 118, 110 and 117 must be closed.

It is obvious that the gas pressure in the anode and cathode circuitsmust differ from atmospheric pressure, otherwise it would not bepossible to observe the pressure variation due to leaks with respect toatmosphere. If the internal pressure is below atmospheric pressure, aleak in the circuit in question results in a rise in pressure, and viceversa.

Furthermore, the recirculating pumps 115 and 125 and the booster pump129 must be shut down so as not to disturb the measurement.

1. A procedure for detecting the state of a fuel cell stack, the fuelcell stack being formed by stacking electrochemical cells each having ananode and a cathode on either side of a polymeric ion-exchange membrane,the fuel cell stack having a fuel gas supply system on the anode side ofthe electrochemical cells and an oxidant gas supply system on thecathode side of the electrochemical cells, wherein the procedurecomprises, upon each shut-down of the fuel cell stack, measuring thedynamic behaviour as the pressure in the anode circuit and the pressurein the cathode circuit evolve and, when said dynamic behaviour showspre-identified characteristic signs, activating a warning signalindicating that the fuel cell stack requires inspection.
 2. Theprocedure for detecting the state of a fuel cell stack according toclaim 1, wherein the measurement of the evolution of the dynamicbehaviour comprises recording the variation of a mathematical functioncombining the pressure in the cathode circuit with the pressure in theanode circuit, and triggering an alarm when the pair of values formed bythe time and the value of the function associated with said time exceedsa predetermined threshold.
 3. The procedure for detecting the state of afuel cell stack according to claim 2, wherein the mathematical functionis the sum of the pressure in the cathode circuit and the pressure inthe anode circuit.
 4. The procedure for detecting the state of a fuelcell stack according to claim 2, wherein the mathematical function isthe average of the pressure in the cathode circuit and the pressure inthe anode circuit.
 5. The procedure for detecting the state of a fuelcell stack according to claim 3, wherein the pair of values is the timethat elapses after the fuel cell stack is considered to be extinguishedand the value of the mathematical function combining the pressure in thecathode circuit with the pressure in the anode circuit associated withsaid time.
 6. The procedure for detecting the state of a fuel cell stackaccording claim 1, wherein pressure variation in said circuits over apredetermined time period t_(c) is measured, a mathematical functioncombining the values of the pressure in said circuits is calculated, anda warning is given when the function is below a warning threshold at theend of a predetermined time period.
 7. The procedure for detecting thesealing state of a fuel cell stack according to claim 1, preceded by aprocedure for shutting down said fuel cell stack, the latter deliveringan electrical voltage to an electrical power line, the shut-downprocedure comprising the following actions: (i) the supply of fuel gasand oxidant gas is cut off; (ii) current continues to be drawn as longas an appropriate indicator indicates that the oxidant gas in theoxidant gas supply system has not been sufficiently consumed; and (iii)nitrogen-enriched gas is injected into the oxidant gas supply system. 8.The procedure for detecting the sealing state of a fuel cell stackaccording to claim 1, for a fuel cell stack comprising an oxidant gassupply system on the cathode side of the electrochemical cells, theoxidant gas supply system comprising at the same time a cut-off valve,placed at the outlet of an oxygen storage tank, and a device for fillingwith pressurized atmospheric air, and a recycling circuit connected tothe outlet of the cathode circuit of the fuel cell stack, together witha water separator, before a return and a connection to the oxidant gassupply line.
 9. The procedure for detecting the sealing state of a fuelstack according to claim 7, wherein the actions (i), (ii) and (iii) areconcomitant.
 10. The procedure for detecting the sealing state of a fuelcell stack according to claim 7, wherein the actions (ii) and (iii) aresuccessive steps, the two actions (i) and (ii) being concomitant. 11.The procedure for detecting the sealing state of a fuel cell stackaccording to claim 7, which further includes, after the action (iii), afuel gas suction step.
 12. The procedure for detecting the sealing stateof a fuel stack according to claim 7, for a fuel cell stack suppliedwith pure oxygen as oxidant, the nitrogen-enriched gas being theatmospheric air.
 13. The procedure for detecting the sealing state of afuel cell stack according to claim 7, wherein the action of interruptingthe fuel gas supply is delayed relative to the action of interruptingthe oxidant gas supply.
 14. The procedure for detecting the sealingstate of a fuel cell stack according to claim 7, wherein the supply ofpure oxygen and the fuel gas supply are interrupted simultaneously. 15.The procedure for detecting the sealing state of a fuel cell stackaccording to claim 7, wherein the current draw is firstly set at a firstlevel, it is then reduced at the same time as certain cells of the fuelcell stack start to drop in voltage, and finally it becomes zero whenthe voltage of the fuel cell stack approaches 0 V.